Melatonin (MLT) is a neural hormone synthesized primarily in the pineal gland and the retina. The hormone is ubiquitous in human tissues being distributed by the blood stream throughout the body. MLT is lipophilic which facilitates its ready penetration through cell membranes. MLT plays an important role in the control of many essential physiological processes. More recently it has been suggested that MLT may function as an anti-oxidant and radical scavenger. The levels of MLT in the blood exhibit a circadian rhythm falling during the daytime and rising at night. Previous studies have shown that in human subjects exposed to electromagnetic fields (EMF) the dark increase in MLT is signficantly reduced. It has therefore been suggested that the biological effects of EMF may be due to a decrease in tissue anti-oxidant capacity resulting from lower MLT levels. The anti-oxidant and radical scavenging properties of melatonin have been inferred from its ability to inhibit reactions thought to involve free radicals. However, little direct evidence exists for the radical-scavenging ability of MLT although it is known that indoles, including tryptophan and its metabolites, can inactivate oxygen radicals and function as anti-oxidants. The object of this research is to examine the reaction of MLT with various active oxygen species including singlet oxygen (1O2), H2O2 and the hydroxyl radical. Both melatonin and its major metabolite 6-hydroxymelatonin (6- HMLT) quenched singlet oxygen. The rate constants in CH3CN were 5.8 x 107 M-1 s-1 and 29 x 107 M-1 s-1 for MLT and 6-HMLT respectively. Under our conditions MLT did not undergo rapid photooxidation suggesting that its interaction with 1O2 is mostly physical in nature. 6-HMLT is a much better quencher of 1O2 than MLT. However, in contrast to MLT, 6-HMLT undergoes photodegradation vi a mechanism that has yet to be determined. The role of 1O2 in this process is unknown.